Quantum Computing

Quantum Computing

That structure became apparent as soon as they decided to forget about electrons or photons, and instead make their qubits at the heart of the atom: the nucleus. Actually, it is the nucleus’s building blocks-protons and neutrons-that have spin. While individual spins tend to pair up and cancel each other out, in some isotopes a few are left over, leaving a net spin in one direction or another.

Nuclear qubits were appealing for several reasons. First, you can make a perfectly fine qubit out of any nuclear isotope that has a spin-as many do. Second, says Gershenfeld, “this is the most coherent system in the universe.” Every nucleus is protected from outside disturbances by its dense cloud of electrons. That means that once you get its spin lined up, it will stay that way for hours or days-an eon in computer time. Third, nuclear qubits are incredibly easy to assemble. “Instead of trying to nanofabricate nanostructures,” says Gershenfeld, “we can just use the ones nature gave us: molecules.” Moreover, he points out, the nuclear spins inside a given molecule tend to interact nicely. Take chloroform, for example, a molecule consisting of a carbon atom attached to three chlorine atoms and one hydrogen atom: When the hydrogen nucleus and the carbon nucleus are spinning the same way, their energy levels will be measurably different from when they are spinning opposite from one another.

Finally, says Gershenfeld, the technology for manipulating these nuclear spins is already very mature. It’s called nuclear magnetic resonance, or NMR, and it is routinely used in chemical analysis and in hospital magnetic resonance imaging scanners. It’s a simple matter to adapt commercial NMR spectrometers to do quantum computing.

Say you want to carry out a logical operation using chloroform-something like, “If carbon is 1, then hydrogen is 0.” You just suspend the chloroform molecules in a solvent, and put a sample in the spectrometer’s main magnetic field to line up the nuclear spins. Then you hit the sample with a brief radio-frequency pulse at just the right frequency. The hydrogen spin will either flip or not flip, depending on what the carbon is doing-exactly what you want for an if-then operation. By hitting the sample with an appropriately timed sequence of such pulses, moreover, you can carry out an entire quantum algorithm-without ever once having to peek at the nuclear spins and ruin the quantum coherence.

So there it was, Chuang and Gershenfeld realized: NMR was a natural for quantum computing. They were not alone. Chuang remembers being “surprised and delighted” to learn that Harvard University NMR expert David Cory and his colleagues were independently making exactly the same proposal. For researchers of Cory’s caliber to enter the field constituted more of an endorsement of the NMR idea than a rivalry, Chuang believed. In any case, there was plenty of work to go around. By this point, Lloyd recalls, with researchers having identified a real application for quantum computing plus a feasible technique for making it work, “all hell was breaking loose” in the field. “All of a sudden there was this wonderful new game to play.”

The game has only gotten better. In just the past few years, for example, Shor and others have shown that quantum computers needn’t be as fragile as researchers once feared; a variety of quantum error-correction schemes will allow the devices to undo the damage caused by environmental perturbations and restore their qubits to full coherence. Lov Grover of Lucent Technologies’ Bell Labs has discovered a quantum search algorithm that is substantially faster than its best classical counterpart. Chuang himself has used NMR to demonstrate Grover’s algorithm, first on a two-qubit quantum computer-a chloroform molecule-and more recently on a three-qubit molecule. Along the way, Chuang and Gershenfeld’s partnership has expanded into a nationwide consortium for quantum computing research, including members from MIT, Stanford, the University of California at Berkeley, IBM and several other industrial partners. Cory’s team, which is working on an NMR demonstration of Shor’s algorithm, has likewise broadened into a Harvard-MIT collaboration. Both groups are among those getting money from the Defense Advanced Research Projects Agency-the arm of the Defense Department that essentially invented the Internet-as part of the first significant federal funding initiative for quantum computing.